Of course, the problem with not reading the thread is that if the resolution were as simple as "log out" there would not be 347 posts here! :-)
For example,you can go back to mine. As a professional programmer, you'll see that I've spent close to 20 hours trying to track this down. My conclusions so far is that it likely has to do with several things. For some, it's random finder icons. For me (and at least Mr. Losada) it's related to .dmg files. I suspect that some part of the OS looks at mounted .dmg files differently than the finder per se. If that part uses low-rez icons (either because the icons are wrong (some bit isn't set, for example) or because the OS thread that looks at icons on .dmg files thinks the monitor resolution is less that it is (my current best guess) then when something is dragged from the mounted .dmg to the hard drive, the lower-rez icon is dragged along. Ipso Facto, the finder later shows that low-rez icon.
I was able to get this to quit being a problem by creating a new user, and -rebooting- (not logging out and in). My test was mounting a .dmg that exhibited the condition. (Checking the finder serves no purpose if my theory is correct, since you're algready installed a low-rez icon, and that won't magically change.)
So, at least for me (and I've been coding for 34 years now) it has come down to:
1) something originating with or at least symptomatic first with .dmg files;
2) something peculiar to a specific user.
As you can read in my previous post, deleting my entire user preference file makes the issue go away for me... so I think I can whittle it down to something in the user prefs. (That would also explain why some people see it, but most don't.)
I suspect now that something we all use (or HAVE used, past-tense) gets into some incorrect preference file and corrupts it. Then, even if that thing we used is no longer present or used, the corrupted prefs continues to supply the OS with incorrect info, and the issue remains.
That would explain why the problem is so difficult to track down, as well.
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During my testing, I did remove all startup items, to no avail, so I'm not confident that a list of what we use will work to solve it... but nonetheless, it can't hurt.
Here's mine:
User login:
chronosyncBackgrounder
Contour ShuttleHelper
TrackballWorksHelper
defaul folder x helper
popchar
FontAgent Activator
Dropbox
~/Library/launchagents
ca.indev.MailTagsHelper.agent.plist
com.adobe.AAM.Updater-1.0.plist
com.adobe.ARM.202f4087f2bbde52e3ac2df389f53a4f123223c9cc56a8fd83a6f7ae.plist
com.adobe.ARM.930da3ce175de4e82bd3cdf1dd8571f74bd3b6a7236bc94bfc00f6e9.plist
com.apple.AddressBook.ScheduledSync.PHXCardDAVSource.2E23BD60-B671-4A2F-82A3-C5E 3CF25993E.plist
com.apple.AddressBook.ScheduledSync.PHXCardDAVSource.67072CA9-8191-4BB2-882C-87B 1AD11D6E3.plist
com.apple.AddressBook.ScheduledSync.PHXCardDAVSource.D8678E11-C6CC-4D3C-9324-E19 A61441824.plist
com.apple.FolderActions.enabled.plist
com.apple.FolderActions.folders.plist
com.c-command.SpamSieve.LaunchAgent.plist
com.google.keystone.agent.plist
com.parallels.desktop.launch.plist
com.valvesoftware.steamclean.plist
ws.agile.1PasswordAgent.plist
Applications running most all the time:
xmarks for Safari
DevonSphereExpress
DragThing
LaunchBar
Fantastical
Mail
KeyboardMaestro
(The following are system Library components, and so are likely NOT involved [since a new user would use these too.])
/Library/launchagents:
com.adobe.AAM.Updater-1.0.plist
com.intego.commonservices.statusitem.plist
com.intego.netupdate.agent.plist
com.intego.task.manager.notifier.plist
com.intego.VirusBarrierX6.alert.plist
com.intego.VirusBarrierX6.statusitem.plist
com.opendns.osx.DNSCryptMenuBar.plist
com.wacom.pentablet.plist
syncmateStarter.plist
/Library/launchDeamons
com.adobe.fpsaud.plist
com.adobe.SwitchBoard.plist
com.apple.SoftwareUpdate.mtversionlog.plist
com.barebones.authd.plist
com.barebones.bbedit.plist
com.bombich.ccc.plist
com.bresink.system.securityagent3.plist
com.cocoatech.pathfinder.SMFHelper6.plist
com.cocoatech.pathfinder.SMFHelper7.plist
com.dymo.pnpd.plist
com.fixamac.spotlessd.plist
com.intego.commonservices.daemon.plist
com.intego.commonservices.icalserver.plist
com.intego.commonservices.metrics.kschecker.plist
com.intego.netupdate.daemon.plist
com.intego.task.manager.daemon.plist
com.intego.VirusBarrierX6.daemon.plist
com.intego.VirusBarrierX6.scanner.daemon.plist
com.microsoft.office.licensing.helper.plist
com.nullvision.noatime.plist
com.opendns.osx.DNSCryptAfterboot.plist
com.opendns.osx.DNSCryptConsoleChange.plist
com.opendns.osx.DNSCryptControlChange.plist
com.opendns.osx.DNSCryptNetworkChange.plist
com.paceap.eden.licensed.plist
com.xrite.device.xrdd.plist
PACESupport.plist
rapiback.plist
/Library/input managers
1PasswdIM
Ecamm
Menu Extra Enabler
/Library/Startup items
EyeConnect
Hptsvr
PACESupport
ProTec6
ProTec6b
SiCoreService